Apparatus and process to condition natural gas for transportation

ABSTRACT

A conditioning assembly to condition a flow of natural gas to reduce hydrate formation for transportation. The conditioning assembly includes a housing with an inlet for introducing a flow of natural gas and an outlet for delivering conditioned gas, wherein the housing is capable of containing natural gas at or above atmospheric pressure. The assembly also includes at least one tube and fin style heat exchanger contained within the housing configured to cool the flow of natural gas to a temperature below hydrate formation temperature and a collection arrangement to remove hydrates from the housing that form during cooling of the natural gas. Also disclosed is a method of conditioning natural gas, substantially corresponding to the assembly described above, wherein a constant flow of natural gas is conditioned by alternating the flow of natural gas between at least two heat exchangers configured in a parallel arrangement.

TECHNICAL FIELD

The present invention relates to a method of conditioning natural gas. In particular, the present invention relates to a method of conditioning natural gas to reduce the formation of hydrate's during the transportation of the gas.

BACKGROUND

Raw natural gas, as it exists when first extracted from a well, typically undergoes processing before being transported. When extracted the natural gas primarily consists of methane, but also includes large amounts of water and often heavier hydrocarbons as well. Changes in pressure or temperature during transportation can result in water and/or hydrocarbons condensing. This is undesirable as the hydrocarbons and water may combine to form solid hydrates. Hydrates are stable solids with an appearance and properties similar to ice. They typically form at temperatures between 0° C. and 8° C., although depending on the gas composition and pressure they can form at temperatures as high as 15° C. Such hydrates arc problematic as they can form blockages in pipes and damage valves and other components, particularly when the natural gas is being transported through a pipe system.

One current method of conditioning natural gas is to pass the gas through process known as dewpoint control. This process may remove heavier hydrocarbons in addition to water, and involves cooling the gas to a temperature slightly below any temperature expected to be encountered in its subsequent handling. During this cooling process, water and hydrocarbons will condense out and are removed from the gas, however the temperature is not reduced to where hydrates form. In subsequent transport and handling of the gas, provided it is not cooled below its dewpoint temperature, no further condensation can occur.

This process, however, assumes that the expected transport temperature is above the hydrate formation temperature. Cold climates can therefore cause problems when the required temperature is low enough to allow hydrates to form. In such a situation it is necessary to either remove the water prior to the dewpoint control process or to add some fluid to act as antifreeze and prevent the formation of hydrates. Addition of antifreeze agents such as methanol or glycol, however, are often uneconomical due to the ongoing costs associated with separating the fluid from the natural gas at a later point and disposal or reprocessing of the fluid.

Currently, the most common method of removing water from natural gas, also known as dehydration, is glycol absorption. The process involves bringing natural gas into contact with glycol, which absorbs the water from the natural gas. The glycol is then removed, regenerated and returned to absorb more water. Another method also commonly used involves a similar process with a solid desiccant instead of glycol. Other processes use pressure difference across membranes.

These methods typically require a large scale processing plant and are therefore not suitable or may be uneconomic for small scale fields or individual wells. A large field of wells are generally connected via pipelines to a single plant. The gas would then often undergo both dehydration and dewpoint control in this single plant before being transported. In such a case, care must be taken to ensure conditions are appropriate to ensure no hydrates form in the pipelines before the gas reaches the processing plant.

The current invention aims at providing a cheaper alternative that is practical to operate at a small scale. This will allow economical processing of natural gas at the well head of individual wells or in small fields where building a full scale dehydration plant is not feasible.

The present invention seeks to provide an improved, or at least an alternative to known, method of conditioning natural gas to reduce hydrate formation during transportation. The present invention also seeks to provide a conditioning assembly to condition a flow of natural gas.

The reference in this specification to any prior publication (or information derived from it), or to any matter which is known, is not, and should not be taken as, an acknowledgement or admission or any form of suggestion that prior publication (or information derived from it) or known matter forms part of the common general knowledge in the field of endeavour to which this specification relates.

BRIEF SUMMARY

In one broad form, the present invention provides a method of conditioning natural gas to reduce subsequent hydrate formation, the method including the steps of:

cooling the natural gas to a temperature at or below the hydrate formation temperature; and

removing from the natural gas hydrates that form during the cooling.

In another form, hydrate formation is reduced during subsequent cooling of the natural gas.

In another form, the subsequent cooling occurs during transportation of the natural gas.

In another form, cooling the natural gas includes introducing a how of natural gas to a heat exchanger wherein the flow of natural gas is cooled via indirect heat transfer.

In another form, the hydrates form on, and are removed from, the surfaces of the heat exchanger.

In another form, water included in the natural gas condenses during cooling.

In another form, the cooling occurs in at least two stages:

a first stage wherein the natural gas is cooled to a temperature where water condenses and is collected; and

a second stage wherein the natural gas is cooled to a temperature below hydrate formation temperature such that hydrates form and are collected.

In another form, the natural gas is cooled in the first stage to a temperature about 3 to 5K above the hydrate formation temperature, wherein the hydrate formation temperature is typically between 0° C. and 8° C.

In another form, the flow of natural gas is ceased in order for the hydrates to be collected.

In another form, the hydrates are collected by warming the heat exchanger such that hydrates melt and the resulting liquid is collected.

In another form, a constant flow of natural gas is conditioned by alternating the flow of natural gas between at least two heat exchangers configured in a parallel arrangement.

In a further broad form, the present invention provides a conditioning assembly to condition a flow of natural gas to reduce hydrate formation for transportation, the conditioning assembly including:

a housing including an inlet for introducing a flow of natural gas and an outlet for delivering conditioned gas capable of containing natural gas at or above atmospheric pressure;

at least one heat exchanger contained within the housing configured to cool the flow of natural gas to a temperature below hydrate formation temperature; and

a collection arrangement to remove hydrates from the housing that form during cooling of the natural gas.

In another form, the heat exchanger is a tube and fin style heat exchanger, wherein the hydrates form on the surfaces of the fins and on the outside of the tubes.

In another form, the conditioning assembly includes at least two zones:

a first zone wherein the natural gas is cooled to a temperature where water condenses and is collected; and

a second zone wherein the natural gas is cooled to a temperature below hydrate formation such that hydrates form and are collected.

In another form, the natural gas is cooled in the first zone to a temperature about 3 to 5K above the hydrate formation temperature, wherein the hydrate formation temperature is typically between 0° C. and 8° C.

In another form, hydrates are removed by ceasing the flow of the natural gas, warming the heat exchanger and collecting the resulting liquid.

BRIEF DESCRIPTION OF FIGURES

The present invention will become more fully understood from the following detailed description of preferred but non limiting embodiments thereof, described in connection with the accompanying drawings, wherein:

FIG. 1 is a schematic of a processing plant including two cooling systems operating in parallel.

PREFERRED EMBODIMENTS

The following modes, given by way of example only, are described in order to provide a more precise understanding of the subject matter of a preferred embodiment or embodiments.

In the foregoing description of preferred embodiments, specific terminology has been resorted to for the sake of clarity. However, the invention is not intended to be limited to the specific terms so selected, and it is to be understood that each specific term includes all technical equivalents which operate in a similar manner to accomplish a similar technical purpose. Terms such as “top” and “bottom”, “front” and “rear”, “inner” and “outer”, “above” and “below”, “upper” and “lower” and the like are used as words of convenience to provide reference points and are not to be construed as limiting terms

In the figures, incorporated to illustrate features of an example embodiment, like reference numerals are used to identify like parts throughout the figures.

In a preferred, but not limiting embodiment, the present invention relates to an apparatus and process of cooling natural gas using heat exchangers and allowing liquid water and solid hydrates to form. The water and hydrates are collected and removed, thereby preventing the formation of hydrates at a later point during transportation. This process and/or apparatus for conditioning natural gas is well suited to small scale operations where building a traditional, large scale processing plant is not feasible.

FIG. 1 shows a schematic of the conditioning process 1. The current embodiment includes a top gas cooler assembly 2 and a bottom gas cooler assembly 3. While the top gas cooler 2 is operating in cooling mode inlet valve 34 and outlet valve 4 are open, allowing gas to flow from the gas inlet 30, through the gas cooler 2 and out the gas outlet 31. Within the gas cooler 2 there are two sections of tube-and-fin coil type of heat exchanger, high temperature 5 and low temperature 6, with cooling fluid from a cooling system 33 passing through the tubes 20, cooling the gas which passes outside the tubes and between the fins 21.

Cooling from the cooling system 33 to the high temperature heat exchanger 5 is controlled so that the gas outlet temperature 7 is approximately 3-5K above the hydrate formation temperature. Cooling from the cooling system 33 to the low temperature heat exchanger 6 is controlled so that the gas outlet temperature 8 is below the hydrate formation temperature.

As the gas is cooled to approximately 3-5K above the hydrate formation temperature, water and some hydrocarbons will condense from the gas (in some instances hydrocarbons may not condense depending on the composition of the gas) Within the high temperature heat exchanger 5, the water and hydrocarbons will condense as liquid and will drain off into the first drain pan 9, from where it is taken away for further processing 10.

Within the low temperature heat exchanger 6, where the gas is cooled below the hydrate formation temperature, hydrocarbons will condense as liquid and drain into the second drain pan 11, but in addition, solid hydrates will form. The intent is to maximise cooling and condensation in the high temperature heat exchanger 5 where the condensate can be removed as liquid, and to minimise the size of the low temperature heat exchanger 6 where hydrate accumulation occurs.

The hydrate will collect on the cooling surfaces of the low temperature heat exchanger 6, outside the tubes 20 and on both sides of the fins 21. As the gas is progressively cooled, as it passes through the low temperature heat exchanger 6, more condensation and hydrate formation will occur throughout the low temperature heat exchanger 6. The spacing of fins 21 and tubes 20 is narrow enough to provide sufficient surface area for cooling, yet wide enough to allow for hydrate build up on the surfaces while still leaving open passages for gas flow. A preferred embodiment may include fins spaced between 3 mm and 50 mm from one another, a more preferred embodiment with fins spaced between 10 mm and 25 mm from one another and a yet more preferred embodiment with fins spaced between 15 mm and 20 mm from one another.

The spacing of tubes 20 and fins 21 may be varied through the low temperature heat exchanger 6, according to the predicted gas cooling profile through the heat exchanger 6. and according to the amount of hydrate which is predicted to form in each part of the low temperature heat exchanger 6. In general, fin 21 spacing is wider at the gas inlet end 7 of the low temperature heat exchanger 6, as hydrate formation is usually greater at the inlet end 7. For example, the fins may be spaced by about 20-30 mm from one another at the inlet end 7 of the low temperature heat exchanger, narrowing to about 4-10 mm at the outlet end 8. In another embodiment, the fins may be spaced by about 25-40 mm at the inlet end 7, narrowing to about 5-10 mm at the outlet end 8.

The exact fin 21 and tube 20 spacing, and its variation through the low temperature heat exchanger 6, may be different for each particular application due to different gas compositions and different required dewpoints. The tube 20 and fin 21 spacing are also varied depending on the required tune for the heat exchanger 6 to remain in service before defrost, and therefore on the amount of accumulated hydrate required to be held without blockage of gas passages.

The high temperature heat exchanger 5 may have tubes 20 and fins 21 spaced much closer together than in the low temperature heat exchanger 6, as there is no need to allow for the build up of hydrates on the surfaces. For example, a preferred embodiment may include fins spaced between 5 mm and 25 mm from one another, a more preferred embodiment with fins spaced between 2 mm and 12.5 mm from one another and a yet more preferred embodiment with fins spaced between 7.5 mm and 10 mm from one another.

The cooling fluid used within the tubes 20 may be a primary refrigerant which boils within the tubes 20, such as propane ammonia or one of the many synthetic refrigerants. Alternatively, it may be a secondary single-phase heat transfer fluid such as water, or a fluid with a lower freezing point such as a mixture of water with glycol or an inorganic salt, or a hydrocarbon. Any other suitable cooling fluid suitable for the purpose may also be used depending on particular requirements for individual installations.

The design of the current system may allow (depending on the particular local conditions and resulting system design) for the cooling to take place using heat exchangers only, not requiring the use of any valves, throttles, pumps or compressors to significantly alter the working pressure or to alter the temperature using pressure changes.

While the top gas cooler 2 is in service, gas flow to the bottom gas cooler 3 is stopped, and the bottom gas cooler 3 is warmed to a temperature which is sufficient to melt off accumulated hydrate. The hydrate melting temperature is not necessarily the same as the hydrate formation temperature. Heat is provided from an available heat source 32, which may include (but is not limited to) waste heat from the cooling system 33.

The warming may be achieved by any of several methods, depending on the application, including (but not limited to):

-   -   circulating a warm fluid from the heat source 32 through the         tubes of the exchanger (either through the same tubes as used         for cooling, or through a separate set of tubes 22 specifically         provided for heating purposes);     -   circulating warm air or warm gas over the outside of the tubes         and between the fins;     -   electric heating elements within the exchanger; and     -   permitting the exchanger to warm naturally by heat gain from the         surrounding atmosphere.

The exact source of heating is not critical; it is the principle of warming the exchanger to melt off hydrate that is important. In the embodiment in FIG. 1, it is shown as circulating a warm fluid from the heat source 32 through a separate set of heating tubes 22 in the low temperature heat exchanger 44 specifically provided for heating purposes.

In some embodiments the hydrates in contact with the heat exchanger will melt first, resulting in hydrates being released from the heat exchanger and collected while still in solid form. In other embodiments, the hydrates may be completely melted and collected in liquid form only. In both cases, the tubes and fins will preferably be arranged to facilitate the flow of hydrates from the heat exchanger surfaces to the second drain pan, such as by angling the fins so that fully or partially melted hydrates flow oil due to gravity. In yet other embodiments, an automatic system to aid in the removal of hydrates may he used, such as a vibration or scraping mechanism, thereby allowing the hydrates to be collected more quickly and reduce the time required for the defrosting mode.

While the top gas cooler 2 is operating in cooling mode, the bottom gas cooler 3 is in defrosting mode. This takes place by closing outlet valve 42 to stop gas flow through the bottom gas cooler 3. Cooling to both the high temperature 43 and low temperature 44 heat exchangers is stopped by closing valves 49, 50 and the low temperature heat exchanger 44 is heated by opening valves 45 to permit warm fluid to flow from the defrost heat source 32 through heating tubes 22. As the low temperature heat exchanger 44 is warmed by the heating tubes 22, the accumulated hydrate melts, and drains off via the second drain pan 46 for further processing 48 outside the gas cooler 1.

The time required to fully melt off all hydrate will vary, depending on the amount of hydrate that has accumulated and the temperature of the heat source, but may be typically 30 minutes to 4 hours. After all hydrate has been melted, the valves 45 are closed to stop heating of the low temperature heat exchanger 44, and cooling is re-started by opening valves 49, 50 connected to the cooling system 33. After the low 44 and high 43 temperature heat exchangers have been cooled again to normal operating temperature for hydrate accumulation, the system is ready for the bottom cooler 3 to be put into gas cooling mode and the top cooler 2 to be taken out of gas cooling mode and the accumulated hydrate melted off.

Changeover from top cooler 2 in gas cooling service to bottom gas cooler 3 in gas cooling service, and vice versa, can be done with no interruption to gas flow and without the outlet 31 gas dewpoint temperature rising above the design point. While the current embodiment only uses two gas cooler assemblies, other embodiments may use three or more if required to meet processing requirements or if greater time is required for the defrosting mode than for the cooling mode. Similarly, a single gas cooler may be used if it is not necessary to provide a continuous supply of conditioned gas.

The heat exchangers are contained within a casing 47, which has a design pressure appropriate to the required operating pressure of the gas cooling process. This may be typically in the range 100 kPa, to 10 MPa_(g), but pressures above or below this range may be used to suit the specific application.

In a further embodiment, the process for conditioning natural gas may further include a heat recovery step which provides a means to reduce the overall cooling duty of the process as herein described. For example, after the natural gas has been cooled to at or below the hydrate formation temperature, it is usually not required at the low temperature during the subsequent transportation of the treated gas. As such a heat recovery step may be included which provides the cool outgoing gas from the process may be used to cool a separate working fluid via indirect heat transfer. Once the separate working fluid is cooled, it may then be used to precool the natural gas introduced to the process as herein described again via indirect heat exchange with the separate working fluid. Accordingly, the energy used to cool the natural gas in the process may be recovered to a degree from the gas leaving the process.

The separate working fluid may be included in a separate working fluid heat recovery circuit where the working fluid is cycled between two heat exchangers by means of a pump. The first heat exchanger is arranged as a precooling step wherein the working fluid precools the natural gas by indirect heat exchange before being introduced to the process as herein described. The working fluid having been heated by the incoming natural gas in the first heat exchanger is then sent to the second of the two heat exchangers in the working fluid heat recovery circuit. The working fluid is then cooled by the outgoing gas again by indirect heat exchange in the second heat exchanger after which the working fluid is cycled back to the first heat exchanger.

Materials of construction of the heat exchanger tubes 20, fins 21 and casing 47 are not restricted, and may be chosen to suit specific requirements, including (but not limited to) considerations of corrosion, strength and thermal performance.

While the embodiments disclosed include only tube and fin type heat exchangers, other suitable heat exchanger designs may be used where appropriate. Similarly, even though the embodiments disclosed use separate low and high temperature heat exchangers, it is envisaged that alternative embodiments could use a single heat exchanger to serve the same purpose, using different regions of the heat exchanger at different temperatures.

While the use of additional stages, such as using a desiccant to further dehydrate the gas, have not been disclosed, there are no functional reasons why this could not be included and such additions are considered to be within the scope of the invention.

Many modifications will be apparent to those skilled in the art without departing from the scope of the present invention. 

1-16. (canceled)
 17. A method of conditioning natural gas to reduce subsequent hydrate formation, the method including the steps of: introducing a flow of natural gas to a heat exchanger; cooling the natural gas to a temperature at or below the hydrate formation temperature; and removing from the natural gas hydrates that form during the cooling, wherein a constant flow of natural gas is conditioned by alternating the flow of natural gas between at least two heat exchangers configured in a parallel arrangement.
 18. The method as claimed in claim 17, wherein hydrate formation is reduced during subsequent cooling of the natural gas.
 19. The method as claimed in claim 18, wherein the subsequent cooling occurs during transportation of the natural gas.
 20. The method as claimed in claim 17, wherein the flow of natural gas is cooled via indirect heat transfer.
 21. The method as claimed in claim 17, wherein the hydrates form on, and are removed from, the surfaces of the heat exchanger.
 22. The method as claimed in claim 21, wherein the heat exchanger is a tube and fin style heat exchanger and hydrates form on the fins of the heat exchanger.
 23. The method as claimed in claim 22, wherein spacing of the fins at an inlet of the heat exchanger is greater than spacing of the fins at an outlet of the heat exchanger.
 24. The method as claimed in claim 20, wherein water included in the natural gas condenses during cooling.
 25. The method as claimed in claim 24, wherein the cooling occurs in at least two stages: a first stage wherein the natural gas is cooled to a temperature where water condenses and is collected; and a second stage wherein the natural gas is cooled to a temperature below hydrate formation temperature such that hydrates form and are collected.
 26. The method as claimed in claim 25, wherein the natural gas is cooled in the first stage to a temperature about 3 to 5K above the hydrate formation temperature, wherein the hydrate formation temperature is typically between 0° C. and 8° C.
 27. The method as claimed in claim 17, wherein the flow of natural gas is ceased in order for the hydrates to be collected.
 28. The method as claimed in claim 27, wherein the hydrates are collected by warming the heat exchanger such that hydrates melt and the resulting liquid is collected.
 29. A conditioning assembly to condition a flow of natural gas to reduce hydrate formation for transportation, the conditioning assembly including: a housing including an inlet for introducing a flow of natural gas and an outlet for delivering conditioned gas capable of containing natural gas at or above atmospheric pressure; a first and a second heat exchanger contained within the housing in a parallel arrangement, each configured to cool the flow of natural gas to a temperature below hydrate formation temperature; and a collection arrangement to remove hydrates from the housing that form during cooling of the natural gas, wherein a constant flow of natural gas is conditioned by alternating the flow of natural gas between the heat exchangers.
 30. The conditioning assembly as claimed in claim 29, wherein the heat exchangers are tube and fin style heat exchangers, wherein the hydrates form on the surfaces of the fins and on the outside of the tubes.
 31. The conditioning assembly as claimed in claim 30, wherein spacing of the fins varies according to expected hydrate formation location.
 32. The conditioning assembly as claimed in claim 29, wherein spacing of the fins at an inlet of the heat exchanger is greater than spacing of the fins at an outlet of the heat exchanger.
 33. The conditioning assembly as claimed in claim 29, wherein the conditioning assembly includes at least two zones: a first zone wherein the natural gas is cooled to a temperature where water condenses and is collected; and a second zone wherein the natural gas is cooled to a temperature below hydrate formation such that hydrates form and are collected.
 34. The conditioning assembly as claimed in claim 33, wherein the natural gas is cooled in the first zone to a temperature about 3 to 5K above the hydrate formation temperature, wherein the hydrate formation temperature is typically between 0° C. and 8° C.
 35. The conditioning assembly as claimed in claim 29, wherein hydrates are removed by ceasing the flow of the natural gas, warming the heat exchanger and collecting the resulting liquid. 